1. Field Of The Invention
This invention relates to an ion mobility spectrometer, and more particularly, to an ion mobility spectrometer having increased sensitivity and especially useful for the detection of involatile compounds such as drugs, explosives and controlled substances.
2. Description Of The Prior Art
This invention relates to ion mobility spectrometry otherwise known as plasma chromatography. The operation of the ion mobility spectrometer is similar to the operation of a time-of- flight mass spectrometer, the difference being that the time-of-flight spectrometer operates in a vacuum (where the mean free path of the contained gases is many times the dimension of the gas container) whereas the ion mobility spectrometer operates at or near atmospheric pressure (where the mean free path of the contained gas is smaller than the dimensions of the container).
The purpose and utility of an ion mobility spectrometer is the identification of one or more gas or vapor constituents in a sample gas and the measurement of the respective concentrations. The sensitivity or limit of detection determines the power of the spectrometer, and much of the prior art is dedicated to various aspects of ion mobility spectrometer sensitivity. Specialized improvements in sensitivity and specificity have been made, but a significant increase in sensitivity is needed to sample involatile (vapor pressures .ltoreq.10.sup.-6 mm Hg) vapors.
A typical ion mobility spectrometer, such as that disclosed by Cohen et al in U.S. Pat. No. 3,621,240 issued Nov. 16, 1971, comprises a reaction chamber and, an ionization source associated with the reaction chamber. The reaction region is conventionally defined as the volume enclosed by the ionization source. The ionization source generates reactant ions which react with a sample contained within the reaction region, thereby forming product ions. The typical ion mobility spectrometer also includes ion drift chamber, an ion injector shutter or grid interposed between the ion reaction chamber and the drift chamber, and an ion collector.
U.S. Pat. No. 4,259,573 which was issued on Mar. 31, 1981 to J. M. Prober et al., describes one improvement for determining small concentrations of chemical compounds by plasma chromatography. Small known increments of species A plus (optionally, another calibrant species) are introduced in turn into a plasma chromatograph together with the unknown sample chemical, and the respective changes in the amplitude or area of the characteristic ion peak of species A in the unknown sample are measured.
U.S. Pat. No. 4,311,669, which was issued on Jan. 19, 1982 to G.E. Spangler and assigned to the assignee herein, described an improvement wherein a sample inlet port of an ion mobility detector has a membrane interface. The sample which is included as a vapor component in a gas stream impinges on the exterior surface of the membrane and penetrates the membrane and is carried into the ion mobility spectrometer by a carrier gas which scrubs the interior surface of the membrane. The membrane serves as both a selection device and barrier which operates to increase the sensitivity and specificity.
U.S. Pat. No. 4,551,624, which was issued on Nov. 5, 1985 to G.E. Spangler et al. and assigned to the assignee herein, describes a different improvement wherein a reagent such as acetone and/or carbon tectrachloride is injected into the carrier gas prior to entering the reaction region. The reagent has a higher proton affinity, electron affinity or acidity than contaminants in the sample gas; a lower proton affinity, electron affinity or acidity than at least one constituent of the sample gas to be detected; and does not cluster with water disposed in the reagent source.
U.S. Pat. No. 4,839,143, issued on June 13, 1989 to K.N. Vora et al. and assigned to the assignee herein, describes a selective ionization source for an ion mobility spectrometer in which an electrolytic reaction with an alkali salt heated to a predetermined temperature reacts with the atoms and molecules in the carrier gas to produce ions with mobilities characteristic of the atoms and molecules. The invention disclosed in this '143 patent is a non-radioactive ionization source and further provides a means for ionization of a broad class of compounds.
In another improvement of ion mobility spectrometers, as described in U.S. Letters Pat. No. 4,732,046 issued on Mar. 22, 1988 to A. H. Lawrence et al., samples are condensed on a sample tube in the form of a hypodermic needle. This sample tube is subsequently heated and the sample is desorbed into a carrier gas which delivers the sample to the ion mobility spectrometer. Some enrichment of the sample concentration may result.
The above cited improvements in the sensitivity of ion mobility spectrometers are important in particular applications. Nevertheless, the improvements fall short of the sensitivity required to detect involatile vapors from say explosives, drugs, pharmaceuticals, and the like; and it would be desirable to provide an ion mobility spectrometer having substantially improved (5 to 6 orders of magnitude) sensitivity.